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    $\begingroup$ very clear. well-deserved +1 $\endgroup$ Commented Jan 23, 2012 at 23:36
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    $\begingroup$ @acl Thanks! I knew there will be many answers, but just could not resist:). I actually consider two of my strongest mma areas to be scoping and performance-tuning, so whenever there is a discussion on those, I go out of control, as you correctly noted some few days ago :) $\endgroup$ Commented Jan 23, 2012 at 23:40
  • $\begingroup$ You mentioned that Module-locale symbols with DownValues don't get garbage collected, which sounded worrying, so I tried Module[{a}, a[1] = 1; ] in a fresh kernel. In this case a (or rather its renamed version) does get garbage collected. Can you elaborate? $\endgroup$ Commented Jan 24, 2012 at 0:00
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    $\begingroup$ I think this if fairly complete, I just would like to mention another example why Block is dangerous: It can incidentally trigger Dynamic evaluations -- and in extreme cases hang or crash the FrontEnd. Evaluate this to see why: Dynamic[x] and then in another cell this: Block[{x},x=1;Pause[1]] $\endgroup$ Commented Jan 24, 2012 at 14:25
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    $\begingroup$ Another comment on the Dynamic example, maybe a little off topic: Would anyone except for me condider it as a bug that the Dynamic[x] will keep showing 1 even after Block[{x},x=1;Pause[1]] has finished and a shift-return-evaluation will show that x has no value defined anymore? Using a Clear[x] will make it update... $\endgroup$ Commented Jan 24, 2012 at 23:51